1. Field of the Invention
The present invention relates to an optical receiving circuit for optical communication and, more particularly, to an optical receiving circuit provided with an optical fiber amplifier.
2. Description of Related Art
In conventional optical receiving circuits provided with an optical fiber amplifier, a pumping semiconductor laser is connected to an erbium-doped optical fiber through an optical multiplexer/branching filter to provide pumping light. A received light signal is amplified by the erbium-doped optical fiber.
The amplified output signal is converted into an electric signal by a photodetector through an optical band-pass filter. Furthermore, the electric signal is amplified by a pre-amplifier and a post-amplifier to be output as an equalizing amplified output signal.
In such arrangements, the receiving optical circuit must control amplification in the erbium-doped optical fiber, because of the detecting range of the photodiode. Amplification control is performed by controlling the injection current supplied to the pumping semiconductor laser. To decide an intensity of the injection current by feedback from the amplified output signal, a part of the equalizing amplified output signal is branched to an input of a peak value detecting circuit where a peak value detection output signal is output therefrom. The detection output signal is inputted into an automatic gain control circuit to control a current value output from a driving circuit for driving the pumping semiconductor laser. The pumping semiconductor laser outputs pumping light by using an injection current supplied by the driving circuit. The automatic gain control circuit is provided to control the injection current from the driving circuit in response to the detection output signal.
The remaining part of the equalizing amplified output signal is output from the optical receiving circuit as a data output signal discrimination and reproduced in a discriminator from a clock output signal extracted by a timing extracting circuit.
In the conventional optical receiving circuit provided with the optical fiber amplifier, the automatic gain control operation is carried out such that the peak value detection output signal has a constant value, and the gain control uses only this peak value detecting signal. Therefore, the automatic gain control characteristic depends on a mark-space ratio, because the mark-space ratio may change independent of the peak value. As a result, there is a problem that the light level of the input light from the optical fiber for amplification to the photodiode for receiving is varied when the mark-space ratio fluctuates. As used herein, it will be understood that the term "mark-space ratio" refers to the ratio of signal pulse width to the signal period including not only a signal pulse but also a following interval. Thus, when the pulses and intervals are of equal width, the mark-space ratio will be 0.5.
In particular, the underestimation of the light level by the mark-space ratio fluctuation may lead to an excessive application of the injection current to the pumping semiconductor laser. In the worst case, it may happen that the received clock cannot be detected due to the breakdown of the photodiode for receiving or the increase of the optical noise caused by the excessive input light.